This invention relates to dimmer and starting circuits for a fluorescent lamp.
Fluorescent lamps characteristically have a minimum current value which must be provided for reliable starting, which value varies with the lamp size. However, the current required for starting a lamp is far in excess of that needed to maintain an arc, once the arc has been struck, i.e., once the lamp is operating. The minimum current required to start a 22 watt circline lamp, for example, is approximately 275 milliamps. But the lamp will operate reliably on currents as low as 125-150 milliamps once the lamp has been started, i.e., once the arc has struck between the opposing lamp filaments. However, with the minimum starting current as the lower limit of current through the loop, the degree of dimming of the lamp is correspondingly limited. Fluorescent lamps also have a maximum current which is determined by the maximum current capacity of the lamp filaments. For example, a 22 watt circline tube can accommodate a maximum current of 875 milliamps. However, the actual maximum current through the lamp during operation, i.e., during light output, is much lower (approximately 600 milliamps) because the current during the pre-heating phase (before arc ignition) is substantially higher than after the arc has struck. The reason is that the impedance of the tube itself, which is in the circuit during operation but not during pre-heating, is on the order of 200 ohms, whereas the resistance of the filaments is much smaller, about 10 ohms. Thus, the impedance of the conventional lamp circuit during operation is much higher than during starting, which results in a lamp brightness during operation lower than what would be produced at the maximum current the filaments can carry. Single level fluorescent lamps as well as multiple-level lamps presently suffer from this same disadvantage. In the case of a single level lamp, the inability to get the desired brightness out of a particular lamp because of the filament current limitation is sometimes addressed by using a longer lamp to obtain the desired brightness. However, there are many fixture applications in which the size of the lamp presents significant design problems. A related disadvantage occurs in conventional fluorescent lamp circuits because the amount of silicon steel in the core of the choke ballast must be sufficient to avoid saturation at the highest electrical loading of the ballast, which occurs during the starting cycle of the lamp. At this time, the bulb or tube itself, as opposed to the lamp filaments, is shunted out of the circuit by the glow bottle starter circuit and thus the higher bulb impedance is not part of the circuit. Since the choke ballast must be designed to be electrically stable during this phase, it must have approximately 30% more magnetic permeability than it requires during operation (when the bulb is part of the circuit impedance).